Heli(aza)cene: A Helical Molecular Tweezer with Tunable Intra- and Intermolecular Charge Transfer

Article abstract of DOI:10.1002/chem.201703781

Non-planar fluorophores offer unique avenues of intra- and intermolecular energy transfer not available in their planar counterparts. We have rationally designed a molecular tweezer based on the pyridine-2,6-dicarboxamide framework having two structurally

Full text of DOI:10.1002/chem.201703781

A Journal of
Accepted Article
Title: Heli(aza)cene: A Helical Molecular Tweezer with Tunable Intra-
and Inter-molecular Charge Transfer
Authors: Aasheesh Srivastava, Rajesh Kumar, Shrivats Semwal, and
Joyanta Choudhury
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Heli(aza)cene: A Helical Molecular Tweezer with Tunable Intra-
and Inter-molecular Charge Transfer
Rajesh Kumar, Shrivats Semwal, Joyanta Choudhury* and Aasheesh Srivastava*[a]
Abstract: Non-planar fluorophores offer unique avenues of intra-
and inter-molecular energy transfer not be available in their planar
counterparts. We have rationally designed a molecular tweezer
based on the pyridine-2,6-dicarboxamide framework having two
structurally similar arms with extended -surface. We have termed
this molecular tweezer as Heli(aza)cene (HAC) due to its
spontaneous adoption of helical conformation stabilized by the
amide and imine moieties present in it. In the helical conformation,
the two arms of HAC are twisted unequally. This asymmetry confers
dissimilar electronic character to the two arms and results in
intramolecular charge transfer interactions in HAC. Homochiral
stacking of the P- and the M- helices in crystal, and profound red-
shifting of the emission at higher concentrations of HAC was
attributed to inter-molecular charge transfer interactions in
aggregated/crystal state. Exposure of HAC, in solution as well as
solid state, to Lewis/Bronsted acids results in rapid and vibrant color
changes. This is the first example of a -layered helical molecule
imine linkage forces the two pyrenes to become spatially
proximal in the helical conformation, which is stabilized through
intramolecular H-bonding between the amide and the imine
residues present in the molecule[10] (Figure 1b). Due to the
presence of extended - surface on the structurally-similar arms
of the molecule, and the spontaneous adoption of helical
conformation stabilized by intramolecular H-bond, we call this
compound Heli(aza)cene or HAC.
exhibiting
tunable
intra-/inter-molecular
charge
transfer
characteristics.
Figure 1. a) Design features of Heli(aza)cene (HAC) (in side 5 is indicating H-
bonding five membered rings). b) The unsymmetrical distortion of the two
arms of HAC in its helical conformation. The electronically dissimilar arms of
HAC are highlighted by red (electron-rich) and blue (electron-poor) colours in
the crystal structure of HAC.
Planar, conjugated molecules have been the mainstay of
traditional fluorescent systems.[1] However, non-planar systems
offer unique characteristics such as chirality and inter-/intra-
molecular charge-transfer as well as a wider diversity in their
packing.[2] For example, helicenes - the helical molecules based
on conjugated, all-carbon framework – show novel chirooptical
and non-linear optical properties.[3] Heteroatom containing
conjugated helical molecules have since been developed in
order to modulate electronic properties of helicenes and also
induce donor/acceptor character to the molecule.[4] The band-
gap in these heteroatom (N, B) containing π-conjugated systems
can be easily tuned by employing Lewis acids/bases, as
demonstrated by the groups of Yamaguchi, Bazan and others.[5]
Azahelicenes-the nitrogen-containing analogues-offer distinctive
optical and opto-electronic properties as well as altered proton
affinities.[6] The nitrogen center in these molecules also acts as
the metal coordination site towards enantioselective catalysis
applications.[3a,7] The charge-transfer bands in Azahelicenes
absorb in the visible region, hence, these molecules are of
interest in energy applications too.[8]
However, we noticed that in the helical conformation, the two
arms of HAC twist differently. This induces dissimilar electronic
character to the two arms of HAC. As a result, the pyrene and
PDA residues within the molecule indulge in intramolecular
charge transfer (ICT) interactions. Similarly, the electronically-
dissimilar pyrene residues on adjacent HAC molecules exhibit
intermolecular
charge
transfer
interactions
(IMCT).
Consequently, HAC exhibits unique optical properties and forms
long, screw-shaped homochiral columns in the crystal state.
Further, the electron-rich imine residues of HAC interact with the
electron deficient species such as Lewis/Bronsted acids to allow
easy tuning of the band gap in HAC.
Bright yellow-colored HAC was readily obtained in excellent
yields by conjugating pyrene-1-carboxaldehyde to N2,N6-bis(2-
amino-phenyl)pyridine-2,6-dicarboxamide (P2, Figure S1, ESI).
HAC, the exclusive product obtained from this reaction, has a
high thermal stability with onset of melting occurring at 331 °C
(Figure S9, ESI). We noted significant differences in the 1H NMR
spectra of the precursor P2 and HAC (in CDCl3, Figure S10,
ESI). For example, the amide protons exhibited considerable
deshielding in HAC when compared to their resonances in P2.
Moreover, as the solution temperature is lowered, the amide
protons further underwent continuous deshielding (Figure S17
and S18, ESI) due to progressive strengthening of the
intramolecular H-bonding with the imine nitrogens.[11] The PDA
protons (Ha and Hb, Figure S10, ESI) became highly shielded in
HAC when compared to in P2. Nuclear overhauser effect (NOE)
studies showed that the amide protons (Hq) have weak
interaction with the C2 protons (Hg) of pyrene that is structurally
quite distant from it (Figure S15, ESI). All this indicated that HAC
In this study, we prepared a helical, fluorescent molecular
tweezer HAC (Figure 1a) by appending pyrene residues to a
conformationally rigid scaffold derived from the pyridine-2,6-
dicarboxamide framework having phenylenediamine (PDA)
groups on either side.[9] The pyrene residues were attached to
the PDAs through imine linkages in order to extend the -surface
of the two arms of the tweezer. The transoid geometry at the
[a]
Rajesh Kumar, Shrivats Semwal, Dr. Joyanta Choudhury and Dr.
Aasheesh Srivastava, Department of Chemistry, Indian Institute of
Science Education and Research, Bhopal, Madhya Pradesh, India-
462066, E-mail: asri@iiserb.ac.in; joyanta@iiserb.ac.in
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201703781
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may be adopting a helical conformation. This was confirmed
upon obtaining crystal structure for HAC as discussed below.
red arm is expected to have higher electron density than the
blue arm. The larger torsional angle between the PDA and
pyrene moieties in the blue arm is expected to decrease the
electronic communication between these two moieties due to
decreased orbital overlap (Figure 1b). Extensive intermolecular
- stacking between the red and the blue pyrene residues of
adjacent HAC is evident in the crystal of HAC (Figure 2a).
These intermolecular interactions are implicated in the formation
of homochiral stacks evident in the crystal structure. These
homochiral stacks are also known to offer dense packing of the
molecules in the crystal structure that is reflected in the higher
density (d = 1.384 g cm-3).[12]
Figure 2. a) Stacking of HAC molecules as homochiral helices in the crystal.
The average separation between the pyrene residues on adjacent molecules
is 3.46 Å. b) Kohn-Sham molecular orbitals of HAC calculated by DFT at the
B3LYP/6-31G (d) level. c) Normalized absorption (blue line) and emission (red
line) spectra of HAC in chloroform (5 μM, λex = 360 nm, slit 3/3) d) Normalized
excitation spectra of HAC in different solvents (monitored at 425 nm for DMF
and DMSO, and at 428 nm for CHCl3, slit 8/8). The intramolecular charge-
transfer band in HAC is highlighted by the star symbol.
The structural rigidity of HAC also imparted to it strong
propensity to crystallize. Upon exposing the chloroform or
dichloromethane solutions of HAC at RT to pentane vapors
yellow crystals were obtained (Figure S19, ESI). Crystallography
studies confirmed that HAC adopts helical conformation in which
the PDA residue on one (blue) arm is coplanar to the central
pyridine ring, while that on the other (red) arm is twisted out of
the plane (Figure S21e, ESI). The pyrene residue attached to
the latter was coplanar to the PDA (red) residue while that
attached to the other arm was not. In the resulting helical
conformation, the PDA residue on the red arm was spatially
proximal to the pyrene residue of blue arm, with a short
interplanar distance of 3.26 Å (Figure 1b). This indicates strong
intramolecular -layer interaction between the PDA and pyrene
residues of different arms. The HAC crystal had equal number of
P- and M- helices (Figure S21d, ESI). However, the presence of
these helices as homochiral stacks along b-axis was noteworthy
(Figure 2a). Overall, HAC crystallised in Pbca space group.
The two structurally similar arms of HAC thus undergo dissimilar
twisting in the helical conformation adopted by the molecule.
The electronic consequences of this dissymetrization were
gauged through DFT calculations. It revealed that the HOMO of
HAC is more localized on the red arm where PDA and pyrene
residues are coplanar, while it is less on the blue arm where
these two residues are non-coplanar. The LUMO is localized
mainly on the blue arm (Figure 2b). So, the intramolecular
electronic interactions between the PDA residue of red arm and
the pyrene residue of the blue arm may also be involved in
stabilizing the helical conformation of HAC. Moreover, the two
arms are electronically dissimilar in the helical conformation –the
Figure 3. a) Normalized concentration dependent emission spectra of HAC (5-
100 μM) in chloroform (λex = 360 nm, slit 3/3, at 298 K, normalized at λmax). b)
Concentration dependent excitation spectra of HAC in chloroform (monitored
at specific emission wavelength of Figure 3a, slit 8/8, and normalized at λmax).
c) Variation of emission (red line) and excitation (blue line) maxima with
concentration of HAC. d) Normalized absorption (blue line) and emission (red
line) of HAC in crystalline state (λex = 470 nm, slit 3/3). Inset of panel d) shows
fluorescence micrograph of HAC (λex = 560 nm, exposure time 50 ms).
Spectroscopic investigations were undertaken subsequently on
HAC. In chloroform, HAC exhibited featureless absorption
bands with broad peaks at 295 and 416 nm (blue line in Figure
2c). Similarly, the fluorescence profile of HAC was also broad –
starting from 380 nm and extending all the way up to 550 nm,
with maximum intensity around 428 nm (red line in Figure 2c).
The excitation spectrum of HAC in chloroform showed a well-
defined peak at 398 nm that is assigned to intramolecular -layer
charge transfer (ICT) between the PDA and the pyrene
residues[13] on opposite arms (Figure 2d). This assignment was
confirmed through synthesis of a reference compound Ref-
imine (Figure S24, ESI) in which such an interaction is
inadmissible. The quantum yield and lifetime of Ref-imine and
HAC are listed in supporting information (Table S2 and Table S3,
ESI). Notably, both lifetime value as well as the fluorescence
quantum efficiency of HAC is significantly lower than those of
Ref-imine (t = 0.48 ns vs 1.35 ns; ФF = 0.002 vs. 0.016
respectively, for HAC and Ref-imine) due to the strong ICT
happening in HAC.[14] The emission and excitation profiles of
HAC in DMF and DMSO solvents were rather different (and
more structured) compared to those in chloroform, indicating
much weaker ICT in these solvents[14c] (Figure 2d, Figure S22,
ESI). We noted that the ICT peak in CHCl3 gained prominence
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as the solution temperature was decreased (Figure S23, ESI).
The enhancement in the ICT peak correlated well with the
strengthening of the intramolecular H-bonding between the
amide and imine residues (see the discussion of NMR studies
above).
The concentration-dependent changes in the fluorescence
profile of HAC in CHCl3 were investigated thereafter (Figure 3a).
At the lowest concentration (5 μM), a broad emission peaking
~425 nm was observed. Upon increasing the concentration of
HAC (from 5 μM to 100 μM), the intensity of fluorescence first
increased till 20 μM, and then decreased subsequently (Figure
S26 and S27, ESI). Further, a continuous red shift in the
emission maximum (from 425 nm to 474 nm) was seen (red line
in Figure 3c). These changes are ascribed to the intermolecular
charge transfer (IMCT) interactions between the electronically
dissimilar pyrene residues on adjacent HAC molecules of the
soluble aggregates.[15] At intermediate concentrations (e.g. 20
μM), the emission profile was bimodal with a peak at 454 nm
along with a shoulder ~430 nm, indicating contributions from
both monomer-like and aggregated forms of HAC (Figure 3a). At
higher concentrations (100 μM), the emission was much weaker,
and the emission maximum was at 474 nm. The emission profile
was also much sharper at this concentration presumably due to
the dominance of aggregated form of HAC. In stark contrast to
the emission profile, the excitation spectra (monitored at the
emission maxima, Figure 3b) showed a continuous blue shift (lex
decreasing from 364 to 350 nm) along with decrease in the
intensity of ICT band at 398 nm (Figure 3b) as the concentration
of HAC was increased (blue line in Figure 3c). This suggests
that HAC forms H-type aggregates and IMCT increases as its
concentration was increased.[16] However, the strong IMCT in
these aggregates cause the overall emission profile to show a
red-shift. It is expected that this aggregation is maximal in the
solid state of HAC. An unstructured absorption and emission
spectrum of HAC in solid state is shown in Figure 3d.[17] The
emission is strongly red-shifted in the solid state (red line Figure
3d), while the excitation shows a blue shift (Figure S28, ESI).
Fluorescence microscopy image of the crystals (Figure 3d)
showed bright-red emission occurring from the crystals when
excited with a green laser (λex = 560 nm). HAC is thus amongst
the limited examples where pyrene-containing molecule shows
luminescence in the red region.[18] Further, the quantum
efficiency (ФF = 0.17) of HAC in the solid state was almost 100
times higher than that in solution state due to restricted
rotational and vibrational freedom in the crystal state.[19]
Figure 4. a) Reversible visual colour change (yellow to red) upon addition of
H+/BF3 to HAC (in CHCl3). b) UV-Vis titration of HAC (10 M in CHCl3) with
BF3. Inset plot shows the change of absorption at two different wavelengths
during this titration. c) HOMO and LUMO of HAC and HAC: 2BF3 complex
estimated by differential pulse voltammetry (DPV). d) Tuning of electronic
properties of HAC through exposure to acids / base.
onset wavelength (λonset) of 480 nm. Incremental addition of BF3
(as its etherate complex) caused continuous decrease (and
subsequent disappearance of) this band (Figure 4b and S32,
ESI). This was accompanied with the formation of a new band
with λmax = 500 nm and λonset = 620 nm. An isosbestic point at
457 nm was also observed during this titration. The binding
stoichiometry between BF3 and HAC was confirmed by Job’s
plot to be 1:2 (Figure S30, ESI). The association constants
between BF3 and HAC (in CHCl3, 298 K), as determined through
UV-vis studies, were K1 = 0.55 M-1 and K2 = 2.5  1011 M-1
(Figure S31, ESI). Differential pulse voltammetry (DPV) studies
revealed the HOMO–LUMO gap of free HAC was 1.87 eV,
which decreased to 1.72 eV after the complex between HAC
and BF3 is formed (Figure 4c). This correlates with the onset
wavelengths of HAC and HAC:2BF3 adduct in UV-vis
spectroscopy (480 nm and 620 nm, respectively). DFT-based
energy optimized structure of HAC and HAC:2BF3 and summary
of the frontier molecular orbital calculations are shown in Figure
S42, ESI.[2d,5a,22] The titration of HAC with BF3 was also
monitored by fluorescence spectroscopy. In the absence of BF3,
the solution of HAC (10 μM) showed an emission band at 433
nm. Upon the incremental addition of BF3, a new emission band
appears at 556 nm. In addition to this, the intensity of the original
band at 433 nm also increased continuously (Figure S44, ESI)
since complexation to BF3 lowers the photoinduced electron
transfer (PET) between the imine and pyrene residues of HAC.
Further, we monitored the excitation spectra for the above two
emission peaks (em at 433 nm and 556 nm, respectively) of
HAC: 2BF3 adduct. During this titration, a broad excitation band
peaking around 486 nm was observed in the case of em = 556
nm (Figure S46, ESI) while no observable change in the
excitation spectra of em = 433 nm was seen (Figure S45,
ESI).[23] The adduct formation could be easily reversed by
exposure to a Lewis base such as ammonia to re-generate the
original color/ UV-vis profile of HAC (Figure S34, ESI).
Electrochemical investigations were undertaken subsequently to
gauge the energy levels of highest occupied molecular orbital
(HOMO) in HAC. The differential pulse voltammetry (DPV) of
HAC in DCM indicated the presence of a low lying HOMO with
energy level of -5.45 eV.[20] Hence, HAC can be expected to
readily donate its electron density to electron deficient species
such as Lewis and Bronsted acids. The basic imine nitrogen of
HAC is expected to be the site for interaction between HAC and
the acid.[21] Indeed, exposure of HAC, in its solution as well as
solid forms, to a range of Lewis and Bronsted acids such as BX3
(X= F, Cl, Br), AlCl3, SnCl4, H2SO4, trifluoroacetic acid etc.,
resulted in a rapid and strong color-change from yellow-to-red
(Figure 4a, Figure S29, 35, ESI). In chloroform, HAC exhibits an
absorption maximum 416 nm (єmax = 6.75  104 M-1 cm-1) with an
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[3]
a) Y. Shen, C.-F. Chen, Chem. Rev. 2012, 112, 1463-1535; b) T.
Verbiest, S. V. Elshocht, M. Kauranen, L. Hellemans, J. Snauwaert, C.
Nuckolls, T. J. Katz, A. Persoons, Science 1998, 282, 913-915; c) Y.
Zhong, B. Kumar, S. Oh, M. T. Trinh, Y. Wu, K. Elbert, P. Li, X. Zhu, S.
Xiao, F. Ng, M. L. Steigerwald, C. Nuckolls, J. Am, Chem. Soc. 2014,
136, 8122-8130; d) M. Rickhaus, M. Mayor, M. Juricek, Chem. Soc.
Rev. 2016, 45, 1542-1556; e) M. Gingras, G. Felix, R. Peresutti, Chem.
Soc. Rev. 2013, 42, 1007-1050.
In summary, we have developed
a novel charge-neutral,
electron-rich molecular tweezer (HAC) having an extended -
surface. The unique positioning of the amide and imine residues
and the presence of bulky pyrene units on each arm of the
molecule forces it to adopt a helical conformation that is
stabilized by the intramolecular H-bond as well as intramolecular
charge transfer interactions. In this conformation, the two arms
of the tweezer are twisted unequally, and hence, they exhibit
altered electronic character. Strong intra- and inter-molecular
charge transfer interactions in solution as well as in solid state
are observed for this compound. Consequently, crystals of this
compound show significantly red-shifted emission. The HOMO-
LUMO band gap of HAC can be further tuned through exposure
to Lewis/Bronsted acids (both in the dissolved as well as the
solid state of HAC). These electron-deficient species bind to the
electron rich imine residue of HAC. This complexation can be
readily reversed upon exposure to Lewis bases such as
ammonia or triethylamine. The precursor P2 of HAC is expected
to be a convenient source for a wide range of helical systems.
Our finding thus opens new avenues towards design of
synthetically convenient molecular tweezers that spontaneously
fold into asymmetric conformations with unique properties. The
nitrogen atoms in HAC are also envisaged to be strong metal
chelators with favourable catalytic consequences.
[4]
a) K. Goto, R. Yamaguchi, S. Hiroto, H. Ueno, T. Kawai, H. Shinokubo,
Angew. Chem. Int. Ed. 2012, 51, 10333-10336; b) M. Miyasaka, M.
Pink, S. Rajca, A. Rajca, Angew. Chem. Int. Ed. 2009, 48, 5954-5957;
c) N. Saleh, C. Shen, J. Crassous, Chem. Sci. 2014, 5, 3680-3694; d)
X.-Y. Wang, X.-C. Wang, A. Narita, M. Wagner, X.-Y. Cao, X. Feng, K.
Müllen, J. Am. Chem. Soc. 2016, 138, 12783-12786; e) K. Shiraishi, A.
Rajca, M. Pink, S. Rajca, J. Am. Chem. Soc. 2005, 127, 9312-9313.
[5] a) G. C. Welch, R. Coffin, J. Peet, G. C. Bazan, J. Am. Chem. Soc. 2009,
131, 10802-10803; b) Z. Zhou, A. Wakamiya, T. Kushida, S.
Yamaguchi, J. Am. Chem. Soc. 2012, 134, 4529-4532.
[6]
a) J. Míšek, F. Teplý, I. G. Stará, M. Tichý, D. Šaman, I. Císařová, P.
Vojtíšek, I. Starý, Angew. Chem. Int. Ed. 2008, 47, 3188-3191; b) T.
Otani, A. Tsuyuki, T. Iwachi, S. Someya, K. Tateno, H. Kawai, T. Saito,
K. S. Kanyiva, T. Shibata, Angew. Chem. Int. Ed. 2017, 56, 3906-3910.
a) M. Akiyama, K. Nozaki, Angew. Chem. Int. Ed. 2017, 56, 2040-2044;
b) L. Norel, M. Rudolph, N. Vanthuyne, J. A. G. Williams, C. Lescop, C.
Roussel, J. Autschbach, J. Crassous, R. Réau, Angew. Chem. Int. Ed.
2010, 49, 99-102; c) V. Vreshch, M. El Sayed Moussa, B. Nohra, M.
Srebro, N. Vanthuyne, C. Roussel, J. Autschbach, J. Crassous, C.
Lescop, R. Réau, Angew. Chem. Int. Ed. 2013, 52, 1968-1972; d) K.
Yavari, P. Aillard, Y. Zhang, F. Nuter, P. Retailleau, A. Voituriez, A.
Marinetti, Angew. Chem. Int. Ed. 2014, 53, 861-865.
[7]
Experimental Section
[8]
[9]
X. Gong, R. M. Young, K. J. Hartlieb, C. Miller, Y. Wu, H. Xiao, P. Li, N.
Hafezi, J. Zhou, L. Ma, T. Cheng, W. A. Goddard, O. K. Farha, J. T.
Hupp, M. R. Wasielewski, J. F. Stoddart, J. Am. Chem. Soc. 2017, 139,
4107-4116.
Synthesis protocols and characterization details (1H, 13C NMR, mass
spectra along with UV-Visible, emission, excitation spectra and single
crystal) for P2, Ref-imine and HAC.
R. Kumar, A. Srivastava, Chem. Eur. J. 2016, 22, 3224-3229.
[10] a) Z. Shirin, J. Thompson, L. Liable-Sands, G. P. A. Yap, A. L. Rheingold,
A. S. Borovik, J. Chem. Soc., Dalton Trans. 2002, 1714-1720; b) Y.
Hamuro, S. J. Geib, A. D. Hamilton, Angew. Chem. Int. Ed. 1994, 33,
446-448.
Acknowledgements
[11] a) V. Berl, I. Huc, R. G. Khoury, M. J. Krische, J.-M. Lehn, Nature 2000,
407, 720-723; b) P. V. Santacroce, J. T. Davis, M. E. Light, P. A. Gale,
J. C. Iglesias-Sánchez, P. Prados, R. Quesada, J. Am. Chem. Soc.
2007, 129, 1886-1887.
This work was supported by funds and facilities provided by
IISER Bhopal. R. Kumar thanks Council of Scientific and
Industrial Research (CSIR), India for Senior Research
Fellowship. Authors thank Mr. Rahul Shukla for initial help with
SCXRD.
[12] R. Amemiya, M. Yamaguchi, Org. Biomol. Chem. 2008, 6, 26-35.
[13] a) Y. Wu, M. Frasconi, D. M. Gardner, P. R. McGonigal, S. T. Schneebeli,
M. R. Wasielewski, J. F. Stoddart, Angew. Chem. Int. Ed. 2014, 53,
9476-9481; b) J. Guasch, L. Grisanti, M. Souto, V. Lloveras, J. Vidal-
Gancedo, I. Ratera, A. Painelli, C. Rovira, J. Veciana, J. Am. Chem.
Soc. 2013, 135, 6958-6967.
Keywords: Heli(aza)cene• H-bonding • charge transfer •
fluorescence • acids
[14] a) Y. Wu, R. M. Young, M. Frasconi, S. T. Schneebeli, P. Spenst, D. M.
Gardner, K. E. Brown, F. Würthner, J. F. Stoddart, M. R. Wasielewski, J.
Am. Chem. Soc. 2015, 137, 13236-13239; b) D. Aumiler, S. Wang, X.
Chen, A. Xia, J. Am. Chem. Soc. 2009, 131, 5742-5743; c) J. Sung, A.
Nowak-Król, F. Schlosser, B. Fimmel, W. Kim, D. Kim, F. Würthner, J.
Am. Chem. Soc. 2016, 138, 9029-9032.
[1]
a) L. Zang, Y. Che, J. S. Moore, Acc. Chem. Res. 2008, 41, 1596-1608;
b) C. Wang, H. Dong, W. Hu, Y. Liu, D. Zhu, Chem. Rev. 2012, 112,
2208-2267; c) T. M. Figueira-Duarte, K. Müllen, Chem. Rev. 2011, 111,
7260-7314; d) J. Massin, W. Dayoub, J.-C. Mulatier, C. Aronica, Y.
Bretonnière, C. Andraud, Chem. Mater. 2011, 23, 862-873.
[15] a) C. Nuckolls, T. J. Katz, L. Castellanos, J. Am. Chem. Soc. 1996, 118,
3767-3768; b) C. Nuckolls, T. J. Katz, G. Katz, P. J. Collings, L.
Castellanos, J. Am. Chem. Soc. 1999, 121, 79-88.
[2]
a) Z. R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 2003, 103,
3899-4032; b) T. Amaya, T. Nakata, T. Hirao, J. Am. Chem. Soc. 2009,
131, 10810-10811; c) M. Gsänger, J. H. Oh, M. Könemann, H. W.
Höffken, A.-M. Krause, Z. Bao, F. Würthner, Angew. Chem. Int. Ed.
2010, 49, 740-743; d) K. Nakano, H. Oyama, Y. Nishimura, S.
Nakasako, K. Nozaki, Angew. Chem. Int. Ed. 2012, 51, 695-699; e) A.
R. Mallia, P. S. Salini, M. Hariharan, J. Am. Chem. Soc. 2015, 137,
15604-15607; f) T. Fujikawa, Y. Segawa, K. Itami, J. Am. Chem.Soc.
2015, 137, 7763-7768; g) S. Pola, C.-H. Kuo, W.-T. Peng, M. M. Islam,
I. Chao, Y.-T. Tao, Chem. Mater. 2012, 24, 2566-2571.
[16]
X. Wei, G. Zhang, Y. Shen, Y. Zhong, R. Liu, N. Yang, F. Y. Al-
mkhaizim, M. A. Kline, L. He, M. Li, Z.-L. Lu, Z. Shao, B. Gong, J. Am.
Chem. Soc. 2016, 138, 2749-2754.
[17] S. K. Park, S. Varghese, J. H. Kim, S.-J. Yoon, O. K. Kwon, B.-K. An, J.
Gierschner, S. Y. Park, J. Am. Chem. Soc. 2013, 135, 4757-4764.
[18] a) S. K. Rajagopal, A. M. Philip, K. Nagarajan, M. Hariharan, Chem.
Commun. 2014, 50, 8644-8647; b) X. Feng, J.-Y. Hu, C. Redshaw, T.
Yamato, Chem. Eur. J. 2016, 22, 11898-11916.
This article is protected by copyright. All rights reserved.

10.1002/chem.201703781
Chemistry - A European Journal
COMMUNICATION
[19] W. Z. Yuan, Y. Gong, S. Chen, X. Y. Shen, J. W. Y. Lam, P. Lu, Y. Lu, Z.
Wang, R. Hu, N. Xie, H. S. Kwok, Y. Zhang, J. Z. Sun, B. Z. Tang,
Chem. Mater. 2012, 24, 1518-1528.
[20]
J. E. Coughlin, Z. B. Henson, G. C. Welch, G. C. Bazan, Acc. Chem.
Res. 2014, 47, 257-270.
[21] G. C. Welch, G. C. Bazan, J. Am. Chem. Soc. 2011, 133, 4632-4644.
[22]
[23]
a) L. Zöphel, V. Enkelmann, K. Müllen, Org. Lett. 2013, 15, 804-807; b)
J.-W. Oh, Y. O. Lee, T. H. Kim, K. C. Ko, J. Y. Lee, H. Kim, J. S. Kim,
Angew. Chem. Int. Ed. 2009, 48, 2522-2524.
K. Matsuo, S. Saito, S. Yamaguchi, J. Am. Chem. Soc. 2014, 136,
12580-12583.
This article is protected by copyright. All rights reserved.

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A helical molecular tweezer that
shows tunable intra- and inter-
molecular charge transfer properties
and profound visual changes, both
in the solution as well as the solid
Rajesh Kumar, Shrivats Semwal,
Joyanta Choudhury and Aasheesh
Srivastava [a]
Heli(aza)cene: A Helical Molecular
Tweezer with Tunable Intra- and
Inter-molecular Charge Transfer
state,
through
exposure
to
Lewis/Bronsted acids is reported.
This article is protected by copyright. All rights reserved.